Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace that has two main sections: a convection heating section and a radiant (or “pyrolysis”) section. The hydrocarbon feedstock typically enters the convection heating section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The resulting products including olefins leave the pyrolysis furnace for further downstream processing, including quenching.
Conventional steam cracking systems have been effective for cracking a high-quality feedstock which contains a large fraction of volatile hydrocarbons, such as gas oil and naphtha. However, steam cracking economics sometimes favor cracking lower cost feedstocks containing resids such as, by way of non-limiting examples, atmospheric residue, e.g., atmospheric pipestill bottoms, and crude oil. Crude oil and atmospheric residue often contain high molecular weight, non-volatile components with boiling points, for example such as in excess of 590° C. (1100° F.). The term resid however generally include the heaviest streams or fractions in a distillation or separation process, e.g., the bottom stream from a vapor-liquid separator or distillation tower, the exact nature of which will depend upon the separation temperatures within the vessel and hence is not necessarily limited merely to one specific cut-off temperature. Over time, portions of the non-volatile components of resid-containing feedstocks lay down as coke in the convection section of conventional pyrolysis furnaces. Only very low levels of non-volatile components can be tolerated in the convection heating section downstream of the point where the lighter components have fully vaporized.
Additionally, cracking heavier feeds, such as kerosenes and gas oils, produces larger amounts of tar, which can lead to rapid coking in the radiant section of the furnace as well as fouling in the transfer line exchangers preferred in lighter liquid cracking service.
To address coking problems, U.S. Pat. No. 3,617,493, which is incorporated herein by reference, discloses the use of an external vaporization drum for the crude oil feed and discloses the use of a first flash to remove naphtha as vapor and a second flash to remove vapors with a boiling point between 230° C. and 590° C. (450° F. and 1100° F.). The vapors are cracked in the pyrolysis furnace into olefins and the separated liquids from the two flash tanks are removed, stripped with steam, and used as fuel.
U.S. Pat. No. 3,718,709, which is incorporated herein by reference, discloses a process to minimize coke deposition. It describes preheating of heavy feedstock inside or outside a pyrolysis furnace to vaporize about 50% of the heavy feedstock with superheated steam and the removal of the residual separated liquid. The vaporized hydrocarbons, which contain mostly light volatile hydrocarbons, are subjected to cracking.
In using a flash drum to separate heavy liquid hydrocarbon fractions containing resid from the lighter fractions which can be processed in the pyrolysis furnace, it is important to effect the separation so that nearly all of the non-volatile components will be in the liquid phase. Otherwise, heavy, coke-forming non-volatile components in the vapor are carried into the furnace causing coking problems.
Increasing the cut in the flash drum, or the fraction of the hydrocarbon that vaporizes, is also extremely desirable because resid-containing liquid hydrocarbon fractions generally have a low value, often less than heavy fuel oil. Vaporizing some of the heavier fractions produces more of the valuable steam cracker feed. This can be accomplished by increasing the flash drum (sometimes referred to as a resid knockout vessel) temperature to increase the vaporized cut. However, the resulting vaporized heavier fractions tend to partially condense in the overhead vapor phase resulting in fouling of the lines and vessels downstream of the flash/separation vessel overhead outlet.
Various patents have attempted to address one or more of the above-mentioned drawbacks, problems, or limitations of the conventional steam cracking process.
For example, U.S. Pat. No. 7,138,047, which is incorporated herein by reference, describes an advantageously controlled process to optimize the cracking of volatile hydrocarbons contained in the heavy hydrocarbon feedstocks and to reduce and avoid coking problems. It provides a method to maintain a relatively constant ratio of vapor to liquid leaving the flash by maintaining a relatively constant temperature of the stream entering the flash. More specifically, the constant temperature of the flash stream is maintained by automatically adjusting the amount of a fluid stream and steam mixed with the heavy hydrocarbon feedstock prior to the flash. The fluid can be water.
U.S. Pat. No. 7,220,887, which is incorporated herein by reference, describes a process for cracking hydrocarbon feedstock containing resid comprising: heating the feedstock, mixing the heated feedstock with a fluid and/or a primary dilution steam stream to form a mixture, flashing the mixture to form a vapor phase and a liquid phase which collect as bottoms and removing the liquid phase, separating and cracking the vapor phase, and cooling the product effluent, wherein the bottoms are maintained under conditions to effect at least partial visbreaking. The visbroken bottoms may be steam stripped to recover the visbroken molecules while avoiding entrainment of the bottoms liquid. An apparatus for carrying out the process is also provided.
U.S. Pat. No. 7,247,765, which is incorporated herein by reference, describes a process for cracking hydrocarbon feedstock containing resid comprising: heating the feedstock, mixing the heated feedstock with a fluid and/or a primary dilution steam stream to form a mixture, optionally further heating the mixture, flashing the mixture within a flash/separation vessel to form a vapor phase and a liquid phase, partially condensing the vapor phase by contacting with a condenser within the vessel, to condense at least some coke precursors within the vapor while providing condensates which add to the liquid phase, removing the vapor phase of reduced coke precursors content as overhead and the liquid phase as bottoms, heating the vapor phase, cracking the vapor phase in a radiant section of a pyrolysis furnace to produce an effluent comprising olefins, and quenching the effluent and recovering cracked product therefrom. An apparatus for carrying out the process is also provided.
U.S. Pat. No. 7,419,584, which is incorporated herein by reference, describes a process for cracking hydrocarbon feedstock containing resid comprising: heating the feedstock, mixing the heated feedstock with a fluid and/or a primary dilution steam stream to form a mixture, optionally further heating the mixture, flashing the mixture within a flash/separation vessel to form a vapor phase and a liquid phase, partially condensing the vapor phase by contacting with a condenser within the vessel, to condense at least some coke precursors within the vapor while providing condensates which add to the liquid phase, removing the vapor phase of reduced coke precursors content as overhead and the liquid phase as bottoms, heating the vapor phase, cracking the vapor phase in a radiant section of a pyrolysis furnace to produce an effluent comprising olefins, and quenching the effluent and recovering cracked product therefrom. An apparatus for carrying out the process is also provided.
U.S. Pat. No. 7,193,123, which is incorporated herein by reference, discloses a process for cracking hydrocarbon feedstock containing resid comprising: heating the feedstock, mixing the heated feedstock with a fluid and/or a primary dilution steam stream to form a mixture, flashing the mixture to form a vapor phase and a liquid phase which collect as bottoms and removing the liquid phase, separating and cracking the vapor phase, and cooling the product effluent. The process comprises at least two of the following conditions: (1) maintaining the bottoms under conditions to effect at least partial visbreaking; (2) reducing or eliminating partial vapor condensation during flashing by adding a heated vaporous diluent to dilute and superheat the vapor; (3) partially condensing the vapor within said flash/separation vessel by contacting with a condenser; (4) decoking internal surfaces and associated piping of the flash/separation vessel with air and steam; (5) utilizing a flash/separation vessel having an annular, inverted L-shaped baffle; and (6) regulating temperature in furnace tube banks used for heating by utilizing a desuperheater and/or an economizer. An apparatus for carrying out the process is also provided.
However, it would be desirable to provide a process for enhancing the conversion of materials in the liquid phase in the knockout drum to materials suitable as non-fouling components for the vapor phase, so as to increase the overall efficiency of the cracking operation.